Nanowire arrays are seeing increasing use in a variety of applications. See, e.g., U.S. Published Patent Application No. 2009/256134. An exemplary silicon nanowire array might consist of a collection of silicon nanowires, on the order of 100 nm in diameter, on the rough order of one micrometer in height, and of approximately cylindrical or frustoconical shape. The axes of the nanowires run approximately parallel to each other. Each is attached at an end to a silicon substrate.
A silicon nanowire array on top of a silicon substrate, can alter the opto-electrical properties of the bulk silicon substrate. For example, a silicon nanowire array may reduce the reflection of the silicon substrate, reduce the reflection at off-angles of incidence, and increase the absorption of the silicon in ways similar to traditional pyramids or light trapping mechanisms used in solar cells.
A common method for growing silicon nanowires is metal-enhanced etching of a silicon-containing substrate. This is discussed, for example, in U.S. Patent Application No. 2009/256134 where further references are provided. In metal-enhanced etching, a metal is deposited on top of a silicon substrate and placed in a solution in which the etch is enhanced wherever the silicon touches metal. Since the metal coverage is not uniform, parts of the silicon are not etched leaving a graded index silicon, cliffs, or nanowires. The metal used can be, for example, gold, platinum, or silver.
Other known processes for obtaining silicon nanowires include reactive ion etching and VLS (vapor-liquid-solid). In VLS, nanowires are commonly grown on a substrate using a metal catalyst and silane.
There is a need to further improve the process by which arrays of silicon nanowires can be formed.